Meeting the energy needs of humans in a sustainable manner is a global challenge. Storage and conversion of energy become increasingly relevant as we move towards greater reliance on renewable or non-traditional energy sources. Devices to store and deliver electricity, in particular, need to be able to efficiently convert chemical energy into electrical energy. Batteries and fuel cells are commercial examples of such devices that are in widespread use today, and that are expected to rise in importance as energy technologies in the future.
Batteries currently suffer from relatively low energy densities, i.e. the quantity of energy (or power) that can be delivered to a user relative to the weight of the device. Higher energy densities in batteries enable longer duration intervals between recharging and increased processing power for portable electronic devices. Furthermore, improved energy densities would enable increased-range electric vehicles and distributed energy-storage.
Improved energy density for batteries can in principle be achieved by increasing the weight fraction of the active materials in battery cells. One way to achieve this is through the use of thicker electrodes. However, these approaches have not been successful. Thick battery electrodes tend to crack under repeated cycling, which electrically isolates the energy-storage material and decreases battery capacity. Conventional electrodes in current state-of-art battery systems such as lithium-ion batteries comprise a film coated onto a metal foil current collector. The film is typically formed from a polymer binder, conductive agents such as carbonaceous materials, and active materials. The active material loadings or the thickness of the electrode film are restricted due to the internal resistance increase of the film, limitations on lithium-ion diffusion, and stress cracking as ions diffuse in and out the electrode, which results in loss of electrical contact and reduced battery capacity.
Two factors limiting the energy density of rechargeable batteries are thus stress cracking of electrodes during cycling and increasing resistance as electrode thickness increases. These two limitations may be overcome by transitioning from a two-dimensional to a three-dimensional current collector that provides fracture-resistant electrical connections across the electrode. One potential solution is to create a three-dimensional current collector from conductive foam that is filled with energy-storage material. This can result in a thicker electrode with all material electrically connected to the foam, minimizing loss of capacity from stress cracking. This solution is hindered, however, because the foam pore volume limits the amount of energy-storage material in the electrode. Additionally, the presence of the foam throughout the electrode creates a weight and power density penalty.
What is needed is an invention to break the current thickness limitations of electrodes, resulting in batteries with greater energy densities. Electrodes are desired that allow lithium ions (or other metal ions, if desired) to diffuse deeper into a thick energy-storage material layer (such as a cathode material), compared to conventional planar electrodes. There is a continuing commercial desired to improve battery electrodes.